It probably just fucks your immune system so you're more likely to catch it. Unfortunately that's enough to create the illusion of efficacy if in those two weeks the vaxxed is counted as unvaxxed (which is how they are typically counted).
Let's say the population is ten people and five get the vaccine and five don't and on average in a two week period one in five would be expected to catch coof but in those two weeks the vaccine doubles the chance of catching coof. First two weeks one person who is not vaxxed and two who are vaxxed (but not yet counted as vaxxed) catch coof. In the second two weeks, it's back to normal and one in each group catches coof. "Wow, four unvaxxed caught coof and only one vaxxed caught coof! 75 percent effective!" I'm oversimplifying but it would actually be even worse because after the first two weeks two of the five vaxxed have actual protection from having caught the real virus.
It probably just fucks your immune system so you're more likely to catch it. Unfortunately that's enough to create the illusion of efficacy if in those two weeks the vaxxed is counted as unvaxxed (which is how they are typically counted).
Let's say the population is ten people and five get the vaccine and five don't and on average in a two week period one in five would be expected to catch coof but in those two weeks the vaccine doubles the chance of catching coof. First two weeks one person who is not vaxxed and two who are vaxxed (but not yet counted as vaxxed) catch coof. In the second two weeks, it's back to normal and one in each group catches coof. "Wow, four unvaxxed caught coof and only one vaxxed caught coof! 75 percent effective!" I'm oversimplifying but it would actually be even worse because after the first two weeks two of the five vaxxed have actual protection from having caught the real virus.